1. Field of the Invention
The present invention relates to a driving circuit for an electroluminescent device that receives a current and emits light (hereinafter referred to as an EL device) and to an active-matrix display apparatus that uses the driving circuit in displaying an image.
2. Description of the Related Art
Recently, a display apparatus using an EL device has attracted attention as a replacement for a display apparatus using a cathode ray tube (CRT) or a liquid crystal display (LCD). In particular, a current-controlled organic EL device, whose emission brightness is controlled by a current passing through the device, is being actively developed. Also being developed is a color display panel that includes many organic EL devices of three colors having different emission wavelengths aligned on a substrate with driving circuits.
A current-writing driving circuit, which is tolerant of variations in characteristics of used thin-film transistor (TFT) elements, is typically employed. In this case, a display signal supplied to a signal line is a current signal. FIG. 8 illustrates an example configuration of a current-writing driving circuit proposed in the specification of U.S. Pat. No. 6,373,454. In FIG. 8, an n-channel drive transistor described in the aforementioned specification is changed to a p-channel one. FIG. 9 is a timing chart of a control signal to be input to a driving circuit illustrated in FIG. 8 via scanning lines P1 and P2.
A driving circuit 1 illustrated in FIG. 8 outputs a drive current corresponding to an input current Idata to an EL device. The input current is received from a signal line “data” at the drain of a transistor M1. The drain of the transistor M1 is an input terminal of the driving circuit 1 for receiving a current signal. The drive current is supplied from the drain of a drive transistor M3 to the EL device. In FIG. 8, the drain of the drive transistor M3 via a switching transistor M4 is an output terminal of the driving circuit 1.
The driving circuit 1 further includes a switching transistor M2 for opening and closing between its drain and the drain of the drive transistor M3. The driving circuit 1 further includes the switching transistors M1 and M4. When the switching transistor M2 is on (it is closed as the switch), the switching transistor M1 is on and guides a signal current of the signal line data to the drain of the drive transistor M3. When the switching transistors M1 and M2 are off, the switching transistor M4 is on and passes a drain current of the drive transistor M3 through the EL device “EL” as a drive current. The switching transistors M1, M2, and M4 are current-path switching units configured to switch the passage of the drain current of the drive transistor M3 between a path for a signal current and a path for a drive current.
The driving circuit 1 is connected to a light emitting power source line PVdd, the signal line data, and the scanning lines P1 and P2. Writing operation and illuminating operation are performed on the driving circuit 1. In writing, P1 is H, P2 is L, the drive transistor M3 is in a diode-connected state, and a signal current Idata supplied from the signal line passes. In accordance with the magnitude of this current, a voltage occurs between the source and the gate, and a storage capacitor C1 is charged.
In illuminating, P1 is L, P2 is H, and the drain terminal of the drive transistor M3 is connected to the current-injected terminal (in this case, the anode terminal) of the EL device. Because the gate of the drive transistor M3 is separated from the drain, a voltage charged in the storage capacitor C1 in writing is maintained which is the gate voltage of the drive transistor M3. A current corresponding to it passes through the drain. The voltage of the storage capacitor C1 depends on a gate-source threshold voltage of the drive transistor M3 and the relationship between the drain current and the drain-source voltage (hereinafter referred to as the current-voltage characteristic). The drain current of the drive transistor M3 determined by the circuit is substantially the same as the signal current Idata because the difference between the threshold voltage and the current-voltage characteristic is negated. Therefore, the EL device is illuminated at brightness corresponding to the signal current Idata.
EL devices exhibit a deterioration phenomenon in which long-time illumination causes a decrease in brightness.
FIG. 4 illustrates a decrease in brightness of an EL device continuously driven at a constant current. The x axis indicates the illumination time, and the y axis indicates the display brightness. When the start time of illumination is 0, the brightness relatively sharply decreases from the initial brightness L0 to the brightness L1 up to the elapsed time T1. After the time T1, the brightness decreases gradually. The period up to the illumination time T1 is called an initial deterioration period, and the period thereafter is called a late deterioration period. For most EL devices, the initial deterioration period is short, so the initial deterioration can be eliminated by the execution of short-term aging. As a result, in many cases, such as a display panel, the late deterioration period starting from the illumination time T1 is used.
The way in which deterioration progresses depends on the length of an elapsed time and the magnitude of a current passing in the elapsed time. Because the degree of deterioration of a pixel illuminated for a long time and that of its surrounding pixel are different, even if a displayed image is switched to one in which these pixels have the same brightness, the long-time displayed image remains as a “burned-in” image which can be seen. In particular, for a digital camera or a portable device, each which have indications for, for example, capturing information, a clock, and various states, are displayed at one fixed position on the screen, so these displayed indications are likely to be “burned-in”. “Burned-in” images are said to be identifiable even with a brightness difference of approximately 2%. Therefore, it is necessary that (L1−L2)/L1 be less than 2% where the product guarantee period is the period from T1 to T2, L1 is the brightness at the time T1 when the initial deterioration period elapses, and L2 is the brightness at the elapsed time T2.
However, it is difficult to make the decrease in the brightness of present EL devices less than 2% because their product guarantee periods vary from approximately to 100,000 hours. As a result, “burn-in” is a serious problem.